An original concerted antarafacial mechanism for the addition of diorganosilyl-zinc reagents across the CC triple bond of ynamides is computationally investigated using DFT calculations. This concerted mechanism, leading to a trans-product in only one step, results in the formation of a Si–C and a Zn–C σ–bond on opposite sides of the π-system. We demonstrate that the mechanism going through a η²-vinyl intermediate and the proposal of a radical chain pathway are energetically unsustainable. The retained concerted antarafacial pathway is tested on experimental selectivities: the regioselectivity, in favor of the silyl β-addition in ynamide, and stereoselectivity, which is cis– with (Me₂PhSi)₂Zn but trans– with [(Me₃Si)₃Si]₂Zn, are well reproduced by DFT calculations. The regio- and stereoselectivity are discussed using the activation strain model and a chemical bonding analysis.

Frédéric Guégan, Fabrice Chemla, Franck Ferreira, Hélène Gérard, Alejandro Perez-Luna,* and Stéphanie Halbert* 

Chemistry – A European Journal, 2024, Volume 30, Issue 43

https://doi.org/10.1002/chem.202401751

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